A development sought in the design of propulsion engines in order to improve their adaptability to the varying operating conditions which occur on a supersonic aircraft depending on the different phases of flight is to provide for a variable operational cycle of the engine. This involves, in particular, reconciling the need to obtain a large thrust per unit flow at supersonic speeds and, consequently, a high ejection velocity for the gases, with the need to reduce this ejection velocity at subsonic speeds, particularly during take-off, in order to limit sound nuisance.
A solution to this problem has already been proposed in EP-A-0 076 192, which involves two connected coaxial propulsive units, an inner one and an outer one. However, this construction comprises two separate combustion chambers and several rotating bodies with concentric shafts, which leads to a fair degree of complexity in manufacture and operation.